1. Field of the Invention
This invention relates to dielectric ceramic compositions which are useful as materials for dielectric resonator.
2. Description of the Prior Art
In recent years, there have been widely used radio apparatus which are employed in frequency ranges of several GHz, e.g. so-called personal radio apparatus and automotive telephones. Dielectric ceramics for use as dielectric resonators which are conventionally built in such radio apparatus are, for example, MgO-CaO-TiO.sub.2 ceramics and BaO-TiO.sub.2 ceramics. The articles obtained from the known ceramics have a specific inductive capacity of 20-40 and a surface roughness, Rmax, of 3-4 .mu.m. The temperature characteristics of resonance frequency of resonators made of these ceramics are in the range of +100 to -100 ppm/.degree.C. A no-load Q value is in the range of about 4000 to 7000. Thus, the ceramics themselves involve little or no practical problems with respect to the characteristics thereof.
However, the known ceramic articles have the serious problem that the insertion loss is great. This is attributed to the fact that a surface resistance, Rs, of a conductor coated on the surface of the ceramic articles is higher by 50 to 60% than a surface resistance, Rso, of a conductor coated on a ceramic article having a smooth surface. In order to lower the surface resistane, Rs, it is necessary to make ceramic articles having a small surface roughness, Rmax. Accordingly, there is a high demand of development of dielectric ceramic compositions which are able to produce articles which are better in surface condition than the known ceramic compositions.